The present invention relates to magnetic coils for producing highly uniform magnetic fields, such as those required for nuclear magnetic resonance imaging, and particularly to apparatus for compensating for inhomogeneity of the magnetic field produced by a superconducting coil.
Nuclear magnetic resonance (NMR) imaging requires that a static magnetic field, B.sub.o, be developed which is relatively homogeneous (having a constant magnitude) throughout a defined volume. In NMR systems for whole body medical diagnostic imaging, the B.sub.o magnetic field strength typically ranges from about 0.04 to about 1.5 Tesla or more. This magnetic field establishes nuclear spin precession distribution in the patient. Subsequent radio frequency radiation from excited atomic nuclei in the patient are received and employed to produce imaging data. If the static magnetic field is significantly inhomogeneous, undesirable artifacts will occur in the image data.
The uniform magnetic field is developed by a main magnetic coil and several active correction coils which are disposed on a cylindrical surface. In an NMR imaging system, these coils are contained in a cryogenic chamber so that they become superconducting. The magnetic field produced by the coils is oriented in an axial direction with respect to the hollow cylinder on which the coils are disposed. The main magnetic coil is designed to produce as uniform a field as is practical. However, even when extraordinary steps are taken to ensure proper construction of the main coil and magnetic field uniformity, some spatial field uniformity errors remain. Accordingly, it is conventional practice to employ relatively low power active correction coils to perturb the state magnetic field from the main coil in a manner which increases the overall field homogeneity.
A method for homogenizing the magnetic field is taught by U.S. Pat. No. 4,680,551. Initially, the main superconducting coil is excited by a current source and then short circuited so that the excitation current continues to flow through the main coil. Then, the magnetic field magnitude is measured throughout the interior volume of the cylinder. The measurements are used by a correction algorithm in a computer to determine the field inhomogeneity and the magnitude of the current necessary to excite each of the correction coils to improve the homogeneity of the field. The results of this computation indicate the magnitude of the excitation current to be applied to the correction coils. Each of the superconducting correction coils is excited and then short circuited. Once the highly homogeneous magnetic field has been so established, the NMR coils are maintained in the superconducting state for months at a time.
All superconducting coils have a small but finite resistance and as a result, the coil currents decay slowly over time. This decay causes a drift in the static magnetic field within the cylindrical volume. The field drift due to the main coil current decay induces additional currents in the correction coils which produces a change in their magnetic flux that opposes the field drift. Consequently, the perceived temporal magnetic field drift of the coil assembly is very slow which is beneficial for NMR imaging where high temporal stability of the static field is desirable.
However, the alteration of the magnetic flux produced by the additional current induced in the correction coils changes their contribution to the correction of the magnetic field from the main coil. Consequently, over a long period, the drift induced currents in the correction coils degrade the homogeneity of the B.sub.o magnetic field within the cylinder. As a result, a service technician must periodically go through the laborious and expensive process of measuring the field throughout the cylinder and re-exciting the correction coils to homogenize the B.sub.o field. Therefore, it is desirable to further compensate for the magnetic field drift to prolong the period between adjustment of the correction coils.